Homologous recombination in mismatch repair inactivated eukaryotic cells

ABSTRACT

A mammalian cell having a mismatch repair-deficient phenotype is provided, where one or both alleles of a gene essential for mismatch repair, such as an Msh gene, are inactivated. Using this cell in a gene knock-out methodology advantageously allows efficient homologous recombination, even when the DNA sequences of the donor and recipient sequences diverge by significantly more than 0.6%.

BACKGROUND OF THE INVENTION

[0001] Introduction

[0002] The introduction of specific modifications in the prokaryotic and eukaryotic genome is a powerful tool in studying gene function both at the level of individual cells and in the context of a complete organism. In addition, modification of specific genes may result in the generation of industrially and medically important organisms, whereas correction of defective alleles in eukaryotic cells may provide a substantial step forward in the development of somatic gene-therapy protocols.

[0003] The method of genetic modification relies on the ability of virtually every cell type to exchange DNA sequences with a high degree of nucleotide sequence similarity by a process which is called homologous recombination (1). Briefly, the method of genetic modification involves the generation of a so-called targeting construct, a DNA sequence that is largely identical to the specific chromosomal locus to be modified, but differing from this locus by specific modifications. These modifications can be as small as the deletion, insertion or substitution of a single base-pair, or be as large as the deletion or insertion of ten's of kilobasepairs. On entry of the targeting construct into the cell, exchange of sequences flanking the modification with their chromosomal counterparts, will result in the introduction of the modification into the recipient chromosome.

[0004] The efficiency of homologous recombination in both prokaryotes and eukaryotes strongly depends on complete sequence identity of exchanging DNA strands (2-5). Thus, sequence dissimilarities as small as 0.5% can already strongly impede homologous recombination. In several bacterial species, Escherichia coli, Salmonella typhimurium and Streptococcus pneumoniae as well as in the yeast Saccharomyces cerevisiae it has been unequivocally shown that the DNA mismatch repair system is responsible for suppressing recombination between homologous but nonidentical DNA sequences (6-8).

[0005] We have now demonstrated that in mammalian cells deficient for the DNA mismatch repair gene Msh2, homologous recombination has lost the requirement for complete sequence identity between exchanging DNA sequences. This finding provides a new method for modifying the eukaryotic genome using DNA targeting constructs which substantially differ from the target locus in the region where recombination takes place by genetically and/or functionally inactivating the cell's mismatch correction system.

[0006] The method obviates the requirement for DNA targeting constructs that are largely identical to the target locus, thus allowing the efficient genetic modification of both somatic cells and the germ line of outbred organisms. This may provide a route to somatic gene therapy and the modification of eukaryotic species of which inbred strains are not easily available. Also, genomic sequences can be replaced by small oligonucleotides carrying one or more basepair alterations or by large chromosomal sequences derived from other species. The method can also be used to generate large deletions by intra- or extrachromosomal homologous recombination between repeated but diverged sequences.

[0007] Genetic Modification of Mice

[0008] The introduction of specific genetic alterations into the germ line of mice was made possible by a combination of new techniques in molecular biology and embryology that have become available during the last ten years (9-11). The method entails the introduction of a planned genetic modification in embryonic stem (ES) cells by homologous recombination. ES cells are originally derived from the inner cell mass of 3.5 day old pre-implantation embryos and can be maintained by in vitro culture as immortalized cell lines, retaining the undifferentiated state under appropriate culture conditions. At present, a number of ES cell lines are available. ES cells injected into the blastocoel of 3.5 day blastocyst-stage embryos can efficiently compete with the inner-cell-mass cells of the recipient blastocyst in embryonic development thus generating a chimeric mouse consisting of cells derived from the recipient blastocyst and the injected ES cells. Should the in vitro modified ES cells contribute to the germ line of chimeric animals, the ES cell genome can be transmitted to the next generation giving animals carrying the introduced modification in all their cells. Intercrossing of such animals reveals the phenotypic consequences of homozygosity of the modified gene.

[0009] The most commonly introduced genetic modification in ES cells is gene disruption leading to gene inactivation. This allows to study gene function by analyzing the consequences of the absence of a particular gene in the context of a complete organism. Also, many hereditary human diseases result from hemizygosity of specific genes [e.g. cancer predisposition syndromes like hereditary nonpolyposis colorectal cancer (12), Li Fraumeni syndrome (13), retinoblastoma (14)]. The generation of mouse models for such diseases by disruption of the mouse homologs of the genes involved provides an invaluable tool for studying hereditary diseases in an experimental setting.

[0010] Gene Disruption in ES Cells

[0011] The inactivation of a specific gene in ES cells via homologous recombination starts with the preparation of a DNA targeting construct (15). Two types of targeting constructs can be used. In a replacement-type vector, a drug-resistance marker gene (conferring to the cell resistance to drugs like G418, Hygromycin B. Puromycin, Histidinol) disrupts a sequence which is homologous to a sequence in or around the target gene in the recipient genome; in an insertion-type vector the marker gene flanks the homologous sequence. The targeting construct is introduced into ES cells by electroporation (or alternatively by Ca-Phosphate precipitation, lyposome-mediated DNA transfer or micro-injection) and cells are selected for stable integration of the targeting construct into the recipient genome by growth in medium containing the appropiate drug. Drug-resistant colonies are the result of either one of two events: integration of the targeting construct at a random site in the genome or integration of the marker gene in the target locus via homologous recombination between the flanking sequences and their chromosomal counterparts. The replacement-type vector integrates by homologous recombination on both sides of the marker gene; the insertion-type vector integrates by a single homologous recombination event leading to duplication of the region of homology. Thus, the marker gene serves two purposes: it allows selection of cells that have taken up the targeting construct and, on integration via homologous recombination, it will disrupt the target gene thereby modifying its function.

[0012] Distinguishing ES cell clones resulting from homologous recombination from those resulting from random integration, requires the DNA of individual clones to be analysed by Southern hybridisation or the polymerase chain reaction.

[0013] Unfortunately, in many targeting experiments, random integration was often found to be far more efficient than homologous recombination and also large variations in targeting efficiency were observed for different genes (16). In this respect, mammalian cells differ from bacteria and lower eukaryotes like yeast (17), Leishmania major (18) or Trypanosoma brucei (19), where integration of exogenous DNA into the recipient genome exclusively or predominantly occurs via homologous recombination. To date, three factors clearly affecting the recovery of homologous recombinants in mammalian cells have been identified. First, the frequency of homologous recombination increases substantially with the total length of the homologous sequences up to 14 kilobase-pairs (20). Second, the expression level of the marker gene at the target locus affects the frequency of recovery of homologous recombinants: low expression may lead to loss of homologous recombinants, whereas high expression may allow selection of homologous recombinants at an elevated drug concentration (21). Third, sequence dissimilarities between the targeting construct and the chromosomal target locus strongly suppress the efficiency of homologous recombination (5).

[0014] High Efficiency Targeting with Isogenic DNA Constructs

[0015] The suppression of homologous recombination in ES cells by small sequence dissimilarities became clear by a gene targeting experiment aimed at disrupting the Retinoblastoma gene with a neomycin resistance marker gene. Two targeting constructs were prepared carrying the neomycin resistance marker embedded in 10.5 kb of Rb sequence. In one construct the Rb sequence was derived from mouse strain 129, and was therefore identical to the corresponding chromosomal locus in the ES cells, which were also derived from mouse strain 129. In the other construct, the Rb sequence was derived from mouse strain BALB/c. The two constructs, designated 129Rb-neo and B/cRb-neo are described in the accompanying publication Proc. Natl. Acad. Sci. USA, Vol. 89, pp. 5128-5132 on pages 5128 and 5129, FIG. 1. The two constructs contained corresponding Rb sequences and were therefore largely similar. However, they differed approximately 0.6% at the nucleotide level (which corresponds to the level of sequence polymorphism found in the human population). Thus, in a stretch of 1687 basepairs that was sequenced, the BALB/c sequence differed from the 129 sequence by 9 basepair substitutions, three small deletions (of 1, 4 and 6 nucleotides) and two polymorphic CA-repeats (see Proc. Natl. Acad. Sci. USA, Vol. 89, pp. 5128-5132, page 5130, FIG. 4). On introduction of these constructs in 129-derived ES cells, homologous recombination at Rb with the 129-derived construct was 50-fold more efficient than with the nonisogenic BALB/c-derived construct. To provide additional evidence that the suppression of recombination was solely dependent on the polymorphisms between the endogenous locus and the targeting DNA, the inverse experiment was performed, i.e. targeting of a BALB/c-derived ES cell line with the 129- and BALB/c-derived constructs. This experiment yielded the inverse result, i.e. a higher targeting efficiency with the BALB/c-derived construct than with the nonisogenic 129-derived construct:

[0016] With a somewhat different targeting construct, consisting of a hygromycin resistance gene embedded in 17 kb of isogenic Rb DNA, we observed that 80% of all Hygromycin B-resistant colonies resulted from homologous recombination (5). This demonstrates that, in the presence of perfect homology, also in mammalian cells homologous recombination rather than random integration can be the predominant event.

[0017] Although clearly not all problems of gene targeting have been solved, many genes have now been successfully targeted by the use of isogenic DNA targeting constructs. However, genetic modification of cells derved from an outbred organism can become a difficult endeavour as isogenic targeting constructs are not easily available. In this case, efficient gene targeting would require the targeting construct to be prepared from DNA derived from the target cell. Especially in the context of gene therapy, this would raise a tremendous practical obstacle to correction of a defective gene. Also, base sequence divergence imposes a major barrier to exchanging a large chromosomal region of one species by the syntenic region of another species. The present invention provides a way to overcome these problems.

[0018] The Introduction of Subtle Mutations

[0019] Although protocols for disruption of genes in inbred ES cell lines (and in somatic cell lines of which isogenic DNA targeting constructs can be prepared) are rather well developed, the introduction of more subtle mutations is not straightforward. Current protocols are variations on a two-step procedure in which first a marker gene is introduced into the target gene followed by replacement of the marker gene by the desired subtle mutation (5). This procedure requires the marker gene to be selectable both for its presence (first step) and its absence (replacement by the subtle mutation).

[0020] Useful marker genes are the Hprt minigene to be used in Hprt-deficient ES cells (positive selection in HAT medium; negative selection by 6-thioguanine) and a combination of the neomycin resistance gene (positive selection by G418) and the Herpes Simplex Virus thymidine kinase gene (negative selection by Gancyclovir). In an alternative procedure, the subtle mutation and the marker gene are present on the same targeting construct and concomittantly introduced into the genome by homologous recombination. If an insertion-type vector was-used, the marker gene can be removed by intrachromosomal recombination between the duplicated sequences that were generated during the first integration event (23). In case of a replacement-type vector, the marker gene can be removed if it was flanked by two site-specific-recombination sites (e.g. loxP sites). Recombination between these sites on introduction into the cell of the loxP-specific recombinase Cre will remove the marker gene from the genome (24).

[0021] Although either of the above mentioned procedures has allowed the subtle modification of a number of genes in ES cells, they are highly demanding as to the generation of appropiate DNA targeting constructs and the culturing of ES cells under various selective conditions. Therefore, an attractive alternative to these procedures might be the use of small single- or double-stranded oligonucleotides (up to 100 bases or basepairs), which are identical to the target locus except for one or several basepair alterations. However, our finding that base sequence dissimilarities as small as 0.6% strongly suppress homologous recombination, may impose a major impediment to using such oligonucleotides for the introduction of subtle genetic modifications. The present invention provides a way to overcome this problem and may allow the subtle modification of cell lines and cells derived from living organisms and temporarily cultured in vitro.

DESCRIPTION OF THE INVENTION

[0022] The manuscript included as annex in this application provides a detailed description of the methodology used in this application and will be published in the journal Cell on Jul. 28, 1995.

[0023] As described above, base sequence dissimilarities as modest as 0.6% impose a strong barrier to efficient homologous recombination in mouse embryonic stem cells (5). Suppression of homologous recombination by small base sequence divergencies was earlier observed in bacteria (E. coli, S. typhimurium and S. pneumoniae), yeast and mouse fibroblasts (2-4). The role of the DNA mismatch correction system in suppressing recombination between homologous but diverged sequences, was most dramatically demonstrated by Radman and coworkers in studying bacterial conjugation between the related but diverged species Escherichia coli and Salmonella typhimurium (6). This process relies on entry of chromosomal fragments of one species into the other and recombination of these fragments with the chomosome of the recipient bacterium. The sequence divergence between the two species is estimated to be 20-30% and therefore the recovery of exconjugants from an interspecies cross is about 2×10⁵ fold lower than from an intraspecies cross. However, the recovery of exconjugants from the interspecies cross increased 3×10³ fold if the recipient bacteria carried an inactivating mutation in either the mutS or mutL gene, both of which being essential for DNA mismatch correction.

[0024] The central protein in DNA mismatch correction in E coil is encoded by the mutS gene (25). It recognizes and binds to base mispairs and small loops of up to four unpaired nucleotides. After binding to heteroduplex DNA, the MutS-DNA complex is bound by the mutL gene product which leads to excision of a tract of single-stranded DNA of up to several kilobases that contains the mispaired nucleotide(s). In this process also the mutU gene product plays a role, whereas the mutH gene product ensures removal of newly synthesized strands rather than parental strands. The repair process is completed by resynthesis of the excised strand and ligation of the remaining nick. Mismatch repair in E. coli is responsible for maintaining genome stability in at least two ways: i) by recognizing and repairing mis- and unpaired nucleotides that occur, respectively, by misincorporation and slippage during DNA replication; ii) by recognizing mismatches occurring in heteroduplexes formed at initial stages of recombination between homologous but not-identical sequences. This may either lead to blocking elongation of heteroduplex formation or dissociation of the heteroduplex thus aborting the recombination reaction. Consequently, E. coli strains defective or either mutS or mutL, have a pleiotropic phenotype: an increased mutation rate, including destabilization of simple-sequence repeats and an increased rate of recombination between homologous but diverged DNA sequences. The latter phenotype is clearly manifested by the efficient recovery of recombinant bacteria resulting from conjugational crosses between the related but diverged species Escherichia coli and Salmonella typhimurium wherein the recipient bacterium was deficient for mutS or mutL (6). Also, the frequency of chromosomal rearrangements by ectopic recombination between diverged sequences is substantially elevated in mismatch repair deficient bacteria (26).

[0025] In many respects, the biochemistry of mismatch repair systems in eukaryotes resembles that of the E. coli mutS,L system. Homologs of both genes have been identified in yeast and mammalian cells. Based on mismatch binding in vitro, and on the mutator and recombinator phenotypes of Saccharomyces cerevisiae mutants, the protein encoded by the yeast MSH2 gene seems to be the functional homolog of MutS (27-29). A homolog of the yeast MSH2 gene was identified in mammalian cells by analysis of a G•T-mismatch-binding activity, positional cloning and PCR amplification of mouse DNA using degenerate primers (30). Similarly, homologs of the E. coli mutL gene were identified in yeast and mammalian cells.

[0026] Interestingly, inherited mutations in human mutS and mutL homologs were recently found to be related to the cancer predisposition syndrome HNPCC (hereditary nonpolyposis colorectal cancer), which is characterized by development of tumors of the proximal colon at early age. In these tumors, mismatch repair is lost, as manifested by destabilization of simple sequence repeats, the replication error-positive (RER⁺) phenotype (12).

[0027] In order to investigate the role of the mammalian Msh2 gene in DNA mismatch repair and to address the role of mismatch repair in maintaining genome stability, we generated an ES cell line carrying a disruption in both copies of the mouse MSH2 gene. This line is disignated dMsh2-9. Its construction is described in the accompanying manuscript: “Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to cancer”, by Niels de Wind, Marleen Dekker, Anton Berns, Miroslav Radman, and Hein te Riele, to be published in Cell, the Jul. 28, 1995 on pages 6, 14, 27 (legend to FIG. 1) and 30 (FIG. 1).

[0028] As demonstrated in the manuscript, the phenotypic consequences of Msh2 deficiency in mouse embryonic stem cells provide clear evidence for an essential role of MSH2 in mammalian DNA mismatch repair. First, Msh2-deficient ES cells lack binding activity to a double stranded 38-mer oligonucleotide carrying a G•T mismatch or an unpaired TG dinucleotide [pages 6, 15, 27 (legend to FIG. 2) and 30 (FIG. 2))]. Second, Msh2-deficient ES cells have a mutator phenotype as evidenced by an at least 150-fold increase in the number of cells resistant to 6-thioguanine, indicating mutational inactivation of the X-linked Hprt gene. Moreover, microsatellite length instability was observed in subclones derived from the Msh2-deficient ES cell line, but not in subclones derived from wild-type ES cells [pages 7, 15, 27 (legend to FIG. 3) and 30 (FIG. 3)]. Third, Msh2-deficient ES cells resisted a 20-fold higher concentration of the methylating agent N-Methyl-N′-Nitro-N-Nitrosoguanidine than wild-type ES cells [pages 7/8, 16, 27 (legend to FIG. 4) and 31 (FIG. 4)]. Fourth, mice bred to homozygosity for the disrupted Msh2 allele, originally generated in ES cells, were highly predisposed to tumorigenesis [pages 9, 14/15, and 29 (Table 2)].

[0029] Thus, disruption of Msh2 leads to inactivation of mammalian DNA mismatch repair.

[0030] By using a gene targeting assay, we demonstrated that it is the mammalian DNA mismatch repair system which is responsible for suppressing homologous recombination between sequences differing as little as 0.6% at the nucleotide level. As described above, homologous recombination at the Rb locus with an isogenic DNA targeting construct was 50-fold more efficient than with a similar, but nonisogenic construct containing on the average 0.6% sequence divergence with respect to the target locus (5). However, in Msh2-deficient ES cells, homologous recombination at Rb with the nonisogenic targeting construct was as efficient as with the isogenic construct [see the accompanying manuscript: “Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to cancer”, by Niels de Wind, Marleen Dekker, Anton Berns, Miroslav Radman, and Hein te Riele [pages 8/9, 16, 28 (Table 1)].

[0031] This experiment demonstrates that it is the mammalian DNA mismatch repair system which is involved in preventing recombination between homologous but diverged DNA sequences.

[0032] This finding provides a method for modifying the mammalian genome but also the genome of any other eukaryotic organism via homologous recombination using DNA targeting sequences that substantially differ from the target locus with respect to the nucleotide sequence, by genetic or functional inactivation of the cell's mismatch repair system.

LITERATURE CITED

[0033] 1. Kucherlapati, R., and Smith, G. R., eds. (1988) Genetic Recombination. American Society for Microbiology, Washington.

[0034] 2. Shen, P., and Huang, H. V. (1986) Genetics 112, 441-457.

[0035] 3. Nassif. N., and Engels, W. (1993) Proc. Natl. Acad. Sci. USA 90, 1262-1266.

[0036] 4. Waldman, A. S., and Liskay, R. M. (1988) Mol. Cell. Biol. 8, 5350-5357.

[0037] 5. te Riele, H., Robanus Maandag, E., and Berns, A. (1992) Proc. Natl. Acad. Sci. USA 89, 5128-5132.

[0038] 6. Rayssiguier, C., Thaler, D. S., and Radman, M. (1989) Nature 342, 396-401.

[0039] 7. Claverys, J. P., and Lacks, S. A. (1986) Microbiol. Rev. 50, 133-165.

[0040] 8. Selva, E., New, L., Crouse, G., and Lahue, R. S. (1995) Genetics 139, 1175-1188.

[0041] 9. Capecchi, M. (1989) Science 244, 1288-1292.

[0042] 10. Frohman, M. A., and Martin, G. R. (1989) Cell 56, 145-147.

[0043] 11. Hogan, B., Beddington, R., Constantini, F., and Lacy, E. (1994) Manipulating the mouse embryo, 2nd edition, Cold Spring harbor Laboratory Press, Plainview, N.Y.

[0044] 12. Modrich, P. (1994) Science 266, 1959-1960.

[0045] 13. Malkin et al. (1990) Science 250, 1233-1238.

[0046] 14. Hansen, M. F., and Cavenee, W. K. (1988) Trends Genet. 4, 125-128.

[0047] 15. Thomas, K. R., and Capecchi, M. R. (1987) Cell 51, 503-512.

[0048] 16. Camerini-Otero, R. D., and Kucherlapati, R. (1990) The New Biologist 2, 337-341.

[0049] 17. Sulston, J. E., and Horvitz, H. R. (1977) Devl Biol. 56, 110-156.

[0050] 18. Cruz, A., and Beverly, S. M. (1990) Nature 348, 171-173.

[0051] 19. Ten Asbroek, A., Ouellette, M., and Borst, P. (1990) Nature 348, 174-175.

[0052] 20. Deng, C., and Capecchi, M. (1992) Mol. Cel. Bioi. 12, 3365-3371.

[0053] 21. Hanson, K. D., and Sedivy, J. M. (1995) Mol. Cel. Biol. 15, 45-51.

[0054] 22. Askew, G., Doetchman, T. and Lingrel, J. (1993) Mol. Cell. Biol. 13, 4115-4124.

[0055] 23. Hasty, P.et al. (1993) Nature 350, 243-246.

[0056] 24. Kilby, N., Snaith, M., and Murray, J. (1993) Trends Genet. 9, 413-421.

[0057] 25. Modrich, P. (1991) Annu. Rev. Genet. 25, 229-253.

[0058] 26. Petit, M. -A. et al. (1991) Genetics 129, 327-332.

[0059] 27. Reenan, R. A. G., and Kolodner, R. (1992) Genetics 132, 975-985.

[0060] 28. Miret, J. J., Milla, M. G., and Lahue, R. S. (1993) J. Biol. Chem. 268, 3507-3513.

[0061] 29. Alani, E., Chi, N. -W., and Kolodner, R. (1995) Genes Dev. 9, 234-247.

[0062] 30. Varlet, I. et al. (1994) Nucl. Acids Res. 22, 5723-5728.

[0063] Summary

[0064] To investigate the role of the presumed DNA mismatch repair (MMR) gene Msh2 in genome stability and tumorigenesis, we have generated cells and mice that are deficient for the gene. Msh2-deficient cells have lost mismatch binding, and have acquired microsatellite instability, a mutator phenotype and tolerance to methylating agents. Moreover, in these cells, homologous recombination has lost dependence on complete identity between interacting DNA sequences, suggesting that Msh2 is involved in safeguarding the genome from promiscuous recombination. Msh2-deficient mice display no major abnormalities, but a significant fraction develops lymphomas at early age. Thus, Msh2 is involved in MMR, controlling several aspects of genome

[0065] Introduction

[0066] Cancer is the endpoint of an evolutionary process in which normal cells acquire full malignancy by cumulative genetic or epi-genetic alterations, each conferring proliferative, invasive or metastatic potential to the cell. In colorectal cancer, at least six independent transforming mutations in oncogenes or tumor suppressor genes are required for a cell to develop into a fully malignant tumor (reviewed by Vogelstein and Kinzler, 1993). This evolutionary process might be dramatically accelerated by the early loss of systems that safeguard genome stability, resulting in an increased rate of mutagenesis and chromosomal rearrangements (Hartwell, 1992 and Loeb, 1994). In support of this hypothesis, a number of hereditary cancer predisposition syndromes are attributed to defects in DNA repair. Two of these are Hereditary Nonpolyposis Colorectal Cancer (HNPCC) and Muir-Torre syndrome, which are characterized by the development of tumors of the proximal colon at early age. Also cancers of other parts of the gastrointestinal tract and of the genitourinary tract are often found (Lynch et al., 1993). Muir-Torre patients are, in addition, prone to cancers of the sebaceous glands and the skin (Cohen et al., 1991). Individuals affected by one of these syndromes have most likely inherited a mutant allele of either one of four mismatch-repair (MMR)-related genes called MSH2 , MLH1, PMS1 or PMS2 (Leach et al., 1993; Bronner et al., 1994; Kolodner et al., 1994; Liu et al., 1994, Nicolaides et al., 1994; Papadopoulos et al., 1994; Kolodner et al., 1995; reviewed by Jiricny, 1994 and by Modrich, 1994). In tumors of these patients, MMR is lost, presumably through inactivation or loss of the wild-type allele (Leach et al., 1993; Nicolaides et al., 1994; Papadopoulos et al., 1994; Hemminki et al. 1994). The mechanisms of loss of MMR, the cause of the apparent tissue specificity of tumorigenesis and the role of loss of MMR in the evolution of a normal cell to fully malignant tumors are still obscure. Moreover, recent data suggest that MMR is frequently lost in many types of sporadic tumors (Han et al., 1993; Parsons et al., 1993; Risinger et al., 1994; Chong et al. 1994; Mironov et al., 1994; Orth et al., 1994; Shibata et al., 1994; Suzuki et al., 1994; Umar et al., 1994b; Wada et al., 1994; Wooster et al., 1994; Liu et al., 1995). This supports the notion that loss of MMR may play a general role in tumor evolution.

[0067] The best studied MMR system with respect to both genetics and biochemistry is the mutSL pathway of Escherichia coli (reviewed by Modrich, 1991). Central in this pathway is the protein encoded by the mutS gene, which binds to base mispairs and to loops of up to four unpaired nucleotides. After binding to the heteroduplex the MutS-DNA complex is bound by the mutL gene product, triggering excision of a tract of single-stranded DNA of up to several kilobases that contains the mismatched nucleotide(s). The repair process is completed by resynthesis of the excised DNA strand and ligation of the remaining nick. MMR in E. coli is responsible for maintaining genome integrity in at least two ways: (i) by recognizing and repairing mispaired and extrahelical nucleotides that occur by, respectively, misincorporation and slippage during replication and (ii) by recognizing mismatches occurring in recombination intermediates between homologous but diverged sequences, leading to abortion of recombination. Consequently, E. coli strains deficient for either mutS or mutL have a pleiotropic phenotype: an increased mutation rate, including destabilization of simple-sequence repeats (also called microsatellites; the ‘Replication Error⁺’, or RER⁺ phenotype) and promiscuity in inter- and intragenomic recombination (the recombinator phenotype, Rayssiguier et al., 1989; Petit et al., 1991).

[0068] In many respects the biochemistry of MMR systems in eukaryotes resembles that of the mutSL system (Holmes et al., 1990; Varlet et al., 1990; Thomas et al., 1991; Fang and Modrich, 1993). Homologs of both mutS and mutL were found in eukaryotes. Based on mismatch binding in vitro and on the mutator and recombinator phenotypes of Saccharomyces cerevisiae mutants, the protein encoded by the yeast MSH2 gene (Reenan and Kolodner, 1992a) seems to be the functional homolog of MutS (Reenan and Kolodner, 1992b; Miret et al, 1993; Alani et al, 1995). In higher eukaryotes (Xenopus laevis, mouse and human) homologs of the yeast MSH2 gene have recently been identified, based on either sequence homology, direct sequencing of the major mismatch-binding protein or by positional cloning (Fishel et al., 1993; Leach et al., 1993; Palombo et al. 1994; Varlet et al., 1994). An apparent difference between MutS and MSH2, is the binding of the eukaryotic protein, when overproduced and purified from E. coli or yeast, to stretches of up to 14 extrahelical nucleotides (so called insertion-deletion-type loops, or IDLs) in vitro (Fishel et al., 1994b; Alani et al., 1995), suggestive of a role for Msh2 in the repair of these loops (Umar et al, 1994). Eukaryotic homologs of the mutL gene, called MLH1, PMS1 and PMS2 have been analyzed in yeast (Kramer et al., 1989a; Kramer et al., 1989b; Strand et al., 1993; Prolla et al., 1994a; Prolla et al., 1994b) and human cells (Li and Modrich, 1995).

[0069] The precise role of these mutS and mutL homologs in MMR and the role of MMR in the maintenance of genome stability in mammalian cells remain to be established. In addition, the causative role of MMR in cellular sensitivity to simple methylating agents, as was suggested by the loss of MMR after selection of cells for tolerance to these agents (Branch et al., 1993; Kat et al., 1993) and by the methylation tolerance of a MMR-deficient tumor cell line (Koi et al., 1994) remains to be explored.

[0070] To investigate the involvement of Msh2 in mammalian MMR and the roles of MMR in maintaining genome stability we have generated mouse Embryonic Stem (ES) cell lines which lack both copies of the Msh2 gene. We show that Msh2-deficient cells lack binding to some specific mismatches, have acquired a mutator phenotype and are tolerant to simple methylating agents. Moreover, these cells have lost the barrier to recombination between homologous but diverged sequences. Taken together, these results suggest that MMR is essential for the preservation of multiple aspects of genome stability in mammalian cells.

[0071] To study the effect of MMR deficiency on the etiology and evolution of cancer we generated mice lacking one or both copies of the Msh2 gene in part or all of their cells. Although heterozygous mice, a model for HNPCC, remain healthy to at least 8 months of age, a significant fraction of both chimeric and fully homozygous Msh2 knock-out mice succumbs to lymphomas. These results provide evidence that loss of MMR-controlled genome stability can accelerate the development of cancer.

[0072] Results

[0073] Targeted Disruption of the Msh2 Gene in Mouse ES Cells

[0074] Heterozygous ES cell lines sMsh2-42 and sMsh2-55, carrying a hygromycin-resistance gene inserted between codons 588 and 589 of one allele of the Msh2 gene, were generated by gene targeting using the construct shown in FIG. 1A. Cell line dMsh2-9, carrying the hyg marker in both alleles of the Msh2 gene, was obtained by selecting cell line sMsh2-55 for duplication of the chromosome carrying the targeted Msh2 allele at an elevated hygromycin B concentration, a method that was previously shown to be effective using the neomycin-resistance marker (Mortensen et al, 1992). Targeting events were detected by Southern blot analysis, using probes flanking both sides of the targeting construct (FIG. 1A). An example of such a Southern blot, demonstrating the status of the Msh2 alleles in cell lines wt-2 (a control cell line with the targeting construct integrated by non-homologous recombination), sMsh2-55 and dMsh2-9 is shown in FIG. 1B. Diploidy of cell lines sMsh2-55 and dMsh2-9 was verified by karyotyping (not shown).

[0075] The hyg marker was inserted, in the antisense orientation, N-terminally of the putative ATP binding site (FIG. 1A) in a region where (in the human gene) truncations were found in many HNPCC tumors (Liu et al., 1994). Moreover, no Msh2 transcript could be detected in ES cell line dMsh2-9 by Northern blotting (not shown), indicating that the hyg marker strongly suppressed Msh2 expression. The genotype of ES cell line dMsh2-9 will therefore be indicated by Msh2−/−.

[0076] Mismatch Binding in Msh2−/− Cells

[0077] Two different mismatch-binding activities have been described in mammalian cell extracts using gel-retardation assays: (i) An activity that recognises G.T mismatches (Jiricny et al., 1988; Stephenson and Karran, 1989; Griffin and Karran, 1993), an extrahelical TG dinucleotide (Aquilina et al; 1994) representing an intermediate in replicational slippage of a microsatellite and, weakly, G.A, G.G, A.C and G.U mismatches (Stephenson and Karran, 1989; Hughes and Jiricny, 1992). (ii) In one cell line, an independent activity was detected that recognizes A.C mismatches and also pyrimidine-pyrimidine mismatches (Stephenson and Karran, 1989). Based on protein sequencing, the purified G.T-binding activity was shown to contain the MSH2 protein (Palombo et al., 1994). To characterize the mismatch-binding properties of the Msh2−/− ES cell line, we have performed gel-retardation assays using cell extracts of wild-type ES cell line wt-2 and Msh2-deficient line dMsh2-9. Extracts were incubated with radiolabelled 38-mer double-stranded oligonucleotides containing either a mismatch, an extrahelical dinucleotide, or a 14-nucleotide insertion-deletion-type loop (IDL), followed by electrophoresis on a nondenaturing polyacrylamide gel. Results of these experiments as shown in FIG. 2 clearly demonstrate binding activity in wild-type cell extracts to a G.T mismatch and to an extrahelical TG dinucleotide. Binding to both oligonucleotides was entirely absent in the extract of the Msh2−/− ES cell line, demonstrating the involvement of the Msh2 protein in this binding activity. As shown before (Hughes and Jiricny, 1992), binding was abolished by the inclusion of ATP in the binding reaction (not shown). We were unable to detect specific binding to G.A, G.G, or A.C mismatches under the conditions used in wild-type or mutant cell extracts. In addition, no Msh2-dependent binding to a 14-nucleotide IDL was observed (FIG. 2).

[0078] Msh2−/− Cells have a Mutator Phenotype

[0079] The Msh2−/− cell line was indistinguishable from the wild-type cell line wt-2 and the parental heterozygous cell line sMsh2-55 with respect to growth and plating efficiency (not shown). To address the role of Msh2 in MMR, we determined the stability of two microsatellite markers in Msh2−/− cell line dMsh2-9 and Msh2+/+ line wt-2. For this purpose, 24 subclones were derived of each cell line, which had undergone approximately 20 cell divisions since its generation. Chromosomal DNA, isolated from each expanded subclone was subjected to the polymerase chain reaction (PCR) using two end-labelled primer pairs (D7Mit17 and D14Mit15; Dietrich et al., 1994). Whereas none of the wt-2 sublines showed any alteration in microsatellite length, clear length alterations in many dMsh2-9 sublines were seen for both microsatellites (8 out of 24 for marker D7Mit17 and 6 out of 24 for marker D14Mit15, FIG. 3). This observation strongly suggests that the Msh2 protein is involved in the repair of slipped replication intermediates. The microsatellite-slippage rate was estimated to be 10⁻² to 10⁻³ per generation.

[0080] We subsequently investigated the effect of loss of the Msh2 gene on the mutation frequency of a functional gene. To this purpose, 6×10⁶ cells of cell lines wt-2 and dMsh2-9 were plated in the presence of 6-thioguanine (6-TG) to select for cells that have lost activity of the X-linked Hprt gene by mutation. Whereas no 6-TG-resistant colonies were seen in the wt-2 culture, 188 resistant colonies were present in the dMsh2-9 culture. This result extends the mutator phenotype of Msh2−/− cells to functional genes.

[0081] Msh2−/− Cells are Tolerant to N-methyl-N′-nitro-N-nitrosoguanidine (MNNG)

[0082] MMR was suggested to mediate hypersensitivity to agents that methylate G residues at the O⁶ position. It is believed that MMR recognises the O⁶-meG.T mispair that occurs after erroneous incorporation of a thymidine nucleotide opposite to O⁶-meG during replication (Griffin et at., 1994). This is supposedly followed by excision and repair synthesis, again incorporating thymidine opposite to O⁶-meG, triggering a new round of MMR. The net result of this cycling between excision and resynthesis will be the presence of single-stranded regions at the site of O⁶-meG residues which will lead to double-stranded gaps when replicated during S phase, resulting in cell death (for reviews, see Karran and Bignami, 1992; Karran and Bignami, 1994). It is predicted that loss of MMR will prevent this process, thus conferring tolerance to simple methylating agents. To test this hypothesis directly, survival of Msh2−/− cell line dMsh2-9. Msh+/− cell line sMsh2-21 and wild-type cell line wt-2 was determined after exposure to a range of concentrations of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG). In the growth medium, O⁶-benzylguanine was included to competitively inhibit endogenous methyltransferase activity that might otherwise remove the methyl groups added by MNNG (Dolan et al., 1990). FIG. 4 shows that the LD₅₀ for MNNG was increased 20-fold in the Msh2−/− cell line compared to the heterozygous and wild-type cell lines, directly proving the involvement of Msh2 in determining cellular sensitivity to MNNG. Heterozygous cell line Msh2-21 displayed no increased tolerance to MNNG.

[0083] Loss of Suppression of Recombination between Diverged Sequences in Msh2−/− Cells

[0084] In E. coli (Shen and Huang, 1986), Drosophila (Nassif and Engels, 1993) and mammalian cells (Waldman and Liskay, 1988, Te Riele et al, 1992), the efficiency of recombination between homologous DNA stretches is highly dependent on their sequence identity. We have previously demonstrated that in a gene targeting assay, homologous recombination at the Retinoblastoma (Rb) locus in ES cells, derived from mouse strain 129, was 50-fold more efficient with a 129-derived targeting construct than with a construct derived from a non-isogenic (BALB/c) mouse strain (Te Riele et al, 1992; Table 1). This construct contained 0.6% base sequence divergence with respect to the isogenic 129 construct. To provide additional evidence that suppression of recombination was solely dependent on the polymorphisms between the endogenous locus and the targeting DNA, we performed the inverse experiment, i.e. targeting of a BALB/c-derived ES cell line with the 129- and BALB/c-derived constructs. This experiment yielded the inverse result, i.e. a higher targeting efficiency with the BALB/c targeting construct than with the non-isogenic 129 targeting construct (Table 1). To investigate whether MMR is responsible for this antirecombination effect, we have repeated the Rb targeting experiment in the (129-derived) ES cell line dMsh2-9. We found that in the Msh2−/− cell line homologous recombination at the Rb locus with the non-isogenic construct was as efficient as with the isogenic construct (Table 1). This experiment demonstrates that Msh2 is involved in preventing homologous recombination between diverged DNA sequences.

[0085] Msh2-Deficient Mice are Viable but Predisposed to Tumorigenesis

[0086] To study the effect of MMR deficiency on the development of cancer, Msh2+/− ES cell lines Msh2-55 and Msh2-42 and Msh2−/− ES cell line dMsh2-9 were used to generate chimeric mice. Both the Msh2 single and double knock-out ES cells gave rise to healthy animals with coat-color chimerism of 20 to 70%. This result suggests that MMR deficiency did not alter the capacity of the ES cells to compete with wild-type cells in chimeric development.

[0087] Msh2+/− chimeras were found to transmit the mutant allele through the germ line resulting in Msh2 heterozygous F1 mice. These mice, which may serve as a model for HNPCC, were healthy up to at least 8 months of age (Table 2).

[0088] Intercrossing of Msh2+/− mice resulted in Msh2−/− mice according to a normal Mendelian distribution; they were healthy at birth and fertile. However, 30% of the Msh2−/− mice developed metastasizing lymphomas of T-cell origin with a peak at 2 months of age (Table 2). In addition, one mouse suffered from generalized histiocytic sarcoma at 3.5 months of age. Extensive histological analyses of these mice did not reveal any other abnormality. Also healthy Msh2−/− mice sacrificed at 2 months of age were histologically normal and had normal B- and T-cell populations as evidenced by FACS analysis.

[0089] Discussion

[0090] Lack of Mismatch Binding in Msh2−/− Cell Extracts

[0091] Whereas wild-type cell extracts display efficient binding to both a G.T mismatch as well as to an extrahelical TG dinucleotide, extracts of the ES cell line Msh2-9 lack this binding (FIG. 2), suggesting that the Msh2 protein is essential for binding to these heteroduplexes. We were unable to detect specific binding activity to other mismatches under the conditions used. This result is compatible with binding studies by others which demonstrate that binding to a G•T mismatch is stronger than to other mismatches (Stephenson and Karran, 1989; Hughes and Jiricny, 1992) whereas the independent A.C binding activity is apparently cell-type specific (Jiricny et al., 1988Stephenson and Karran, 1989). The methylation-tolerant cell lines ‘clone B’ and RajiF12 were also found to lack binding to both a G.T mismatch (Branch et al., 1993; Griffin et al., 1994) and to an extrahelical TG dinucleotide (Aquilina et al., 1994). Binding to an A.C mismatch was unaffected (Branch et al. 1993). Therefore, these cell lines may have lost the Msh2 gene. Purified human MSH2, overproduced in E. coli, and purified yeast MSH2 were shown to bind to G.T mismatches (Fishel et al., 1994a; Alani et al., 1995), but also strongly to IDLs of up to 14 nucleotides (Fishel et al., 1994b; Alani et al., 1995). Surprizingly, we could not detect Msh2-dependent binding to a 14 nucleotide IDL in ES cell extracts. We are unable to explain this apparent discrepancy. In contrast to the binding that we observed in ES cell extracts and that of the GTBP protein, the MSH2-containing G.T mismatch-binding protein fraction purified from HeLa cells (Hughes and Jiricny, 1992; Palombo et al., 1994), the binding capacity of the purified MSH2 proteins was not abolished by ATP (Fishel et al., 1994b; Alani et al., 1995). This suggests that the purified MSH2 proteins behave differently from the naturally produced protein. The finding that in Msh2−/− cells length shifts in microsatellites were never larger than 4 nucleotides (representing two CA dinucleotides; FIG. 3), suggests that binding activity of Msh2 to extrahelical loops longer than 4 nucleotides is not required to maintain microsatellite stability. We are currently initiating in vitro repair assays using wild-type and Msh2−/− cell extracts to further elucidate this point.

[0092] Mutator Phenotype of Msh2-Deficient Cells

[0093] Cell line dMsh2-9 had acquired microsatellite instability (the RER⁺ phenotype), indicating failure to repair slipped replication intermediates. The frequency of microsatellite instability was similar to that of some RER⁺ tumor cell lines (Parsons et al., 1993: Bhattacharyya et al., 1994). The mutator phenotype of these cells also affects functional genes as evidenced by the rate of Hprt deficiency in a dMsh2-9 culture which was increased from undetectable (i.e. lower than 1.6×10⁻⁷) for wild-type cells to 3×10⁻⁵. This frequency is in agreement with that found in some RER⁺ tumor cell lines (Bhattacharyya et al., 1994; Shibata et al., 1994; Eshleman et al., 1995), although cell lines that have been selected for MMR deficiency using a methylating agent tend to have somewhat lower mutation frequencies (Goldmacher et al., 1986; Branch et al., 1993; Kat et al., 1993; Aquilina et al., 1994). Growth rate and plating efficiency of Msh2-deficient, heterozygous and wild-type cells were indistinguishable, suggesting that the mutator phenotype does not interfere with viability of the cells. This is remarkable, since both were seriously affected in some cell lines that were selected for MMR deficiency using methylating agents (Goldmacher et al., 1986). This suggests that in the latter cells, additional events may have taken place that interfere with cell viability. Based on the mutator phenotype of Msh2-9 cells and on the binding studies described above, we conclude that Msh2 is involved in, and essential for, postreplicational MMR.

[0094] Sensitivity to Simple Methylating Agents is Mediated by MMR

[0095] Sensitivity of cells to agents that methylate DNA at the O⁶ position of G residues has been, attributed to futile attempts of MMR to correct the resulting O⁶-meG.T basepairs (see results). This hypothesis is supported by two observations: (i) Prolonged selection of cells for tolerance to Methylnitrosourea or MNNG frequently results in loss of MMR (Kat et al., 1993) and loss of G.T-binding activity (Branch et al., 1993; Griffin et al., 1994). (ii) An RER⁺ tumor cell line was found to be tolerant to these agents and sensitivity was restored by introduction of the chromosome containing a wild-type allele of the defective mismatch-repair gene (Koi et al., 1994). Our finding that Msh2-deficient cell line dMsh2-9 had acquired tolerance to MNNG provides direct evidence in support of this hypothesis. This finding may have important clinical implications. The triazene class of carcinostatic drugs probably derives its cytotoxic effect from methylation at the O⁶ position of G residues (Michejda et al., 1994). Whereas MMR-deficient tumors are supposedly tolerant to these drugs, they may elicit severe cytotoxity in normal tissues of the patient. In addition, the persistence of O⁶-meG.T mispairs in MMR-deficient tumor cells might lead to increased G to A transitions (Aquilina et al., 1993), resulting in acceleration of tumor progression. Finally, the finding that simple methylating agents select, or induce, MMR deficiency in cells has led to the hypothesis that this process is involved in determining the tisssue specificity of tumorigenesis in HNPCC patients, due to the presence of such agents in the intestine (Karran and Bignami, 1994; Varlet et al., 1994).

[0096] MMR Edits Homologous Recombination in Mammalian Cells

[0097] Previously, it has been shown that the presence of less than 1% nucleotide difference can strongly suppress homologous recombination in mammalian cells (Waldman and Liskay, 1988, Te Riele et al., 1992). Here, we have extended these results and we have demonstrated that Msh2−/− cells have lost this heterology-dependent suppression of recombination. Based on results obtained in E. coli (Worth et al., 1994), this function is probably mediated by the Msh2 protein, by recognizing mismatches that occur during heteroduplex formation between two not perfectly complementary DNA strands. This may directly destabilize the heteroduplex or, alternatively, prevent elongation of strand exchange. Similar results have recently been obtained with MSH2-deficient yeast strains (Alani et al., 1994; Selva et al., 1995), suggesting that this role of MMR is strongly preserved during evolution. In E. coli, loss of MMR has been shown to result in a high frequency of inter- and intrachromosomal rearrangements due to ectopic recombination between diverged sequences (Rayssiguier et al., 1989; Petit et al., 1991). More than the E. coli genome, mammalian genomes contain many repeated but diverged sequences. Examples of these are gene families. SINE sequences, LINE sequences and pseudogenes (Brosius and Gould, 1992; Nowak, 1994). Ectopic recombination between these sequences would lead to genomic rearrangements that may either be detrimental for cell viability due to loss of essential genomic information or, alternatively, lead to activation of oncogenes or inactivation of tumor suppressor genes. (Bouffler et al., 1993). Homologous recombination between identical DNA stretches should be allowed to proceed, however, to allow recombinational repair of double strand breaks using the (identical) sister chromatid as sequence donor. For this reason, mechanisms counteracting homologous recombination when both recombining stretches contain divergencies, while permitting recombination to proceed when both stretches are identical, may be of great importance for the mammalian cell to enable repair without risking genome rearrangment. Based on the effect of Msh2-deficiency on gene targeting, it is likely that MMR plays this scrutinizing role.

[0098] Our finding that Msh2 acts not only anti-mutagenic but also anti-recombinogenic, suggests that MMR deficiency may contribute to tumorigenesis by elevating the rate of both point mutations and chromosomal rearrangements.

[0099] Role of Msh2 Deficiency in Tumorigenesis

[0100] Msh2-deficiency did not interfere with mouse embryonic development and is compatible with adult life. This observation substantiates the recent finding, that some HNPCC patients seem largely MMR deficient in normal cells (Parsons et al., 1995). A significant fraction of chimeric and pure Msh2−/− mice developed metastasizing lymphomas. It is likely that the developing immune system in newborn mice, in which a high turnover of maturing T cells exists, offers a window where MMR deficiency can strongly accelerate the accumulation of transforming events.

[0101] The basal level of mutagenesis in the cell, estimated to be approximately 10⁻¹⁰ per nucleotide per generation, is considered to be insufficient to sustain the multistep clonal outgrowth of cells in tumorigenesis (Loeb, 1994). Therefore an increased mutation rate, such as caused by MMR deficiency, is likely to be obligatory to accelerate this process (Loeb, 1994). Our results suggest that a rapid cell expansion is required to unveil the effect of MMR deficiency on tumor development. Corroborating this notion, recent studies of microsatellite instability during cancer development show that loss of MMR probably does not occur as the initial step, but rather within an early tumor stage such as an adenoma (Young et al., 1993; Aaltonen et al., 1994; Chong et al., 1994; Shibata et al., 1994; Wada et al., 1994), which, in HNPCC patients, have been found to evolve rapidly into carcinomas (Lynch et al., 1993; Jass et al., 1994). Moreover, two recent studies support the notion that enhanced mutagenesis due to MMR deficiency is responsible for tumor progression: (i): Some RER⁺ tumors had acquired frameshift mutations in the β₂-Microglobulin gene within a CT repeat, indicative of unrepaired replication slippage, and putatively leading to escape from immune surveillance (Bicknell et al., 1994); (ii): in some RER⁺ colon carcinomas from HNPCC patients, p53 and APC were found to contain either frameshifts in repeated nucleotides or G to A transitions, indicative of the failure to repair slipped simple sequence repeats and G.T mismatches, respectively (Lazar et al., 1994).

[0102] In conclusion, our studies on Msh2 deficiency prove the involvement of Msh2 in MMR and show that MMR functions as a mechanism safeguarding the mammalian genome against mutation by misincorporation and recombination between diverged sequences. In addition, our results lend support to the hypothesis that MMR deficiency can strongly accelerate malignant transformation of rapidly expanding cell populations.

[0103] Acknowledgements

[0104] We thank Fina van der Ahé, Net Bosnie, Tania Maidment, and Auke Zwerver for animal care; Loes Rijswijk and Martin van der Valk for pathological examinations; Heinz Jacobs and Paul Krimpenfort for FACS analyses: Roderick Beijersbergen and Eric Kriek for helpful discussions, Dr. A. Pegg for the generous gift of O⁶-benzylguanine and Dr. S. Rastan for kindly providing BALB/c ES cells; Carla Schipper for technical assistance and René Bernards and Piet Borst for critically reading the manuscript.

[0105] Experimental procedures

[0106] Disruption of Msh2 in Mouse ES Cells

[0107] Genomic Msh2 fragments were obtained by screening a 129OLA-derived genomic DNA library with a murine Msh2 cDNA probe (Varlet et al., 1994). The targeting construct was prepared by subcloning a 12.5 kbp BamH I fragment and inserting a hygromycin resistance gene (from PGKhyg, te Riele et al., 1990) into the unique SnaB I site located within an exon sequence of Msh2. Cloning procedures were performed according to Sambrook et al. (1989).

[0108] The targeting construct was separated from vector sequences by gel electrophoresis, purified by electro elution and introduced into 129OLA-derived ES cell line E14 by electroporation as described (te Riele et al., 1992). Electroporated cells were seeded onto gelatin-coated 10-cm dishes (10⁷ cells per plate) and subjected to hygromycin B selection (150 μg per ml) in BRL-conditioned medium (Hooper et al., 1987) the following day. After 10 days, individual hygromycin B-resistant colonies were randomly picked and expanded on mouse embryonic fibroblasts feeder layers. DNA was extracted from expanded colonies, digested with EcoR I and analyzed by Southern hybridization using probes flanking both sides of the targeting construct (FIG. 1A). Two cell lines were obtained out of 135 hygromycin B-resistant colonies showing bands diagnostic for correct integration of the hyg marker between codons 588 and 589 of one copy of the Msh2 gene. These cell lines were designated sMsh2-42 and sMsh2-55. One cell line obtained from this experiment, designated wt-2, carrying a randomly integrated hyg gene, was used as an Msh2+/+ control.

[0109] To obtain an ES cell line carrying a disruption in both copies of Msh2, 10⁶ cells of cell line sMsh2-55 were plated onto 10-cm plates and cultured in BRL-conditioned medium containing 1.0 or 1.5 mg per ml of hygromycin B. After 12 days of of culturing in selective medium and 7 days in non-selective medium, 24 colonies were obtained which had survived 1.5 mg per ml of hygromycin B. DNA was extracted from expanded colonies, digested with EcoR I and analyzed by Southern hybridization. One cell line, designated Msh2-9, carried two disrupted Msh2 alleles, had lost the wild-type copy and contained the normal number of chromosomes. ES cell lines which had survived 1.0 mg per ml of hygromycin B still contained one disrupted and one wild-type Msh2 copy. One of these Msh2+/− lines was designated sMsh2-21 and used for several experiments.

[0110] Generation of Msh2 Mutant Mice

[0111] Chimeric mice were obtained by injecting 10-15 cells of ES cell lines sMsh2-55, Msh2-42 and Msh2-9 into C57B1/6 blastocysts. Male chimeras obtained with Msh2^(+/−) ES cells were crossed with wild-type 129OLA and FVB mice and found to transmit the mutated Msh2 allele through the germ line. Homozygous Msh2 mutant mice were obtained by intercrossing F1 heterozygotes.

[0112] Gel Shift Assay

[0113] Preparation of cell extracts, annealing of oligonucleotides, binding of cell extracts to duplex oligonucleotides containing mismatched or extrahelical nucleotides, and nondenaturing polyacrylamide gelelectrophoresis were performed essentially as described (Stephenson and Karran, 1989). However, gelelectrophoresis was performed in TAE buffer rather than in TBE buffer. To obtain duplex oligonucleotides, the oligonucleotide U: 5′-GGGAAGCTGCCAGGCCCCAGTGTCAGCCTCCTATGCTC-3′ (sequences were derived from Aquilina et al., 1994) was radiolabelled and annealed with any of the following unlabelled oligonucleotides: L-G.T: 5′-GAGCATAGGAGGCTGACATTGGGGCCTGGCAGCTTCCC-3′ (resulting in a G.T mismatch); L-G.A: 5′-GAGCATAGGAGGCTGACAATGGGGCCTGGCAGCTTCCC-3′ (resulting in a G.A mismatch); L-G.G: 5′-GAGCATAGGAGGCTGACAGTGGGGCCTGGCAGCTTCCC-3′ (resulting in a G.G mismatch); L-A.C: 5′-GAGCATAGGAGGCTGACACCGGGGCCTGGCAGCTTCCC-3′ (resulting in an A.C mismatch); L-TG: 5′-GAGCATAGGAGGCTGACACTGTGGGGCCTGGCAGCTCCC-3′ (resulting in an extrahelical TG dinucleotide); L-HOM: 5′-GAGCATAGGAGGCTGACACTGGGGCCTGGCAGCTTCCC-3′ (resulting in a homoduplex); L-LOOP14: 5′-GAGCATAGGAGGCTGACACATACGTGAGTACTCTGGGGCCTGGCAGCTTCCC-3′ (resulting in an IDL loop of 14 extrahelical nucleotides). In all assays, a twofold excess of unlabelled homoduplex competitor oligonucleotide was included. As a positive control, a duplex oligonucleotide containing the binding site for the E2F family of transcription factors was used (Beijersbergen et al., 1995).

[0114] PCR Amplification of Microsatellites

[0115] Subclones of ES cell lines Msh2-9 and wt-2 were generated by seeding cells onto mouse embryonic fibroblasts feeder layers at a density of 10³ cells per 10 cm². At that time, the ES cell lines were in culture for approximately 20 divisions since their generation. Twenty-four colonies from each cell line were expanded. Chromosomal DNA was isolated and subjected to the polymerase chain reaction using two end-labelled primer pairs (D14Mit15 and D7Mit17, Dietrich et al., 1994). Amplified products were electrophoresed on a denaturing polyacrylamide gel.

[0116] Mutation Frequency

[0117] 6×10⁶ cells of ES cell lines dmsh2-9 and wt-2 were placed onto 486 cm² gelatin-coated tissue culture surface in BRL-conditioned medium. After two days, 6-Thioguanine was added at a concentration of 10 μg per ml. After two weeks the number of resistant colonies was counted.

[0118] Sensitivity to MNNG

[0119] ES cell lines dMsh2-9, sMsh2-21 and wt-2 were seeded onto MEF feeder layers at a density of 103 cells per 4 cm² The following day, cells were exposed for one hour to MNNG ranging in concentration from zero to 36.45 μM in serum-free medium. After 4 days of incubation, cells were trypsinized, stained with Trypan blue and counted. During the whole procedure from one hour before exposure to MNNG, O⁶-benzylguanine (20 μM, kindly provided by A. Pegg) was included in the medium in order to competively inhibit endogenous methyltransferase activity that might other vise remove the methyl groups added by MNNG (Dolan et al, 1990).

[0120] Rb Targeting with Isogenic and Non-Isogenic DNA

[0121] Targeting and subsequent analysis of the Rb locus in a BALB/c-derived ES cell line (kindly provided by S. Rastan) and 129OLA-derived ES cell lines dMsh2-9 were performed essentially as described (te Riele et al, 1992). The targeting constructs 129Rb-neo (derived from the 129OLA genome) and B/cRb-neo (derived from the BALB/c genome) carry an insertion of the pMC1neo marker in exon 19 of the Rb gene and differ approximately 0.6% at the nucleotide level (te Riele et al, 1992).

REFERENCES

[0122] Aaltonen, L. A., Peltomaki, P., Mecklin, J. -P., Jarvinen, H., Jass, J. R., Green. J. S., Lynch, H. T., Watson, P., Tallqvist, G., Juhola. M., Sistonen, P., Hamilton, S. R., Kinzler, K. W., Vogelstein, B., and de la Chapelle, A. (1994) Replication errors in benign and malignant tumors from hereditary nonpolyposis colorectal cancer patients. Cancer Res. 54, 1645-1648.

[0123] Alani, E., Chi, N. -W., and Kolodner, R. (1995). The Saccharomyces cerevisiae Msh2 protein specifically binds to duplex oligonucleotides containing mismatched DNA base pairs and insertions. Genes Development 9, 234-247.

[0124] Alani, E., Reenan, R. A. G., and Kolodner, R. D. (1994). Interaction beween mismatch repair and genetic recombination in Saccharomyces cerevisiae. Genetics 137, 19-39.

[0125] Aquilina, G., Biondo, R., Doglotti, E., and Bignami, M; (1993). Genetic consequences of tolerance to methylation DNA damage in mammalian cells. Carcinogenesis 14, 2097-2103.

[0126] Aquilina, G., Hess, P., Branch, P., MacGeoch, C., Casciano, I., Karran, P., and Bignami, M. (1994). A mismatch recognition defect in colon carcinoma confers DNA microsatellite instability and a mutator phenotype. Proc. Natl. Acad. Sci. USA 91, 8905-8909.

[0127] Beijersergen, R. L., Carlée, L., Kerkhoven, R. M., and Bernards, R. (1995). Regulation of the retinoblastoma protein-related p107 by G1 cyclin complexes. Genes Development in press.

[0128] Bhattacharyya, N. P., Skandalis, A., Ganesh, A., Groden, J., and Meuth, M. (1994). Mutator phenotypes in human colorectal carcinoma cell lines. Proc. Natl. Acad. Sci. USA. 91, 6319-6323.

[0129] Bicknell, D. C., Rowan, A., and Bodmer, W. F. (1994). β₂M-microglobulin gene mutations: a study of established colorectal cell lines and fresh tumors. Proc. Natl. Acad. Sci. USA 91, 4751-4755.

[0130] Bouffler, S., Silver, A., and Cox, G. (1993). The role of DNA repeats and associated structures in genomic instability and neoplasia. BioEssays 15, 409-412.

[0131] Branch, P., Aquilina, G., Bignami M., and Karran, P. (1993). Defective mismatch binding and a mutator phenotype in cells tolerant to DNA damage. Nature 362, 652-654.

[0132] Bronner, C. E., Baker, S. M., Morrison, P. T., Warren, G., Smith, L. G., Lescoe, M. K., Kane, M., Earabino, C., Lipford, J., Lindblom, A., Tannerg{dot over (a)}rd, P., Bollag, R. J., Godwin, A. R., Ward, D. C., Nordenskjøld, M., Fishel, R., Kolodner, R.; and Liskay, R. M. (1994). Mutation in the DNA mismatch repair gene homologue hMLH1 is associated with hereditary non-polyposis colon cancer. Nature 368, 258-261.

[0133] Brosius, J., and Gould, S. J. (1992). On “genomenclature”: a comprehensive (and respectful) taxonomy for pseudogenes and other “junk DNA”. Proc. Natl. Acad. Sci. USA 89, 10706-10710.

[0134] Chong, J. -M., Fukayama, M., Hayashi, U., Takiawa, T., Koike, M., Konishi, M., Kiuchi-Yanoshita, R., and Miyaki, M. (1994). Microsatellite instability in the progression of gastric carcinoma. Cancer Research 54, 4595-4597.

[0135] Cohen, P. R., Kohn, S. R., and Kurzrock, R. (1991). Association of sebaceous gland tumours and internal malignancy: the Muir-Torre syndrome. Am. J. Med. 90, 606-613.

[0136] Dietrich, W. F., Miller, J. C., Steen, R. G., Merchant. M., Damron, D., Nahf, R., Gross, A., Joyce, D. C., Wessel, M., Dredge, R. D., Marquis, A., Stein, L. D., Goodman, N., Page, D. C., and Lander, E. C. (1994). A genetic map of the mouse with 4,006 simple sequence length polymorphisms. Nature Genetics 7, 220-240.

[0137] Dolan, M. E., Moschel, R., and Pegg, A. E. (1990). Depletion of mammalian O⁶-alklylguanine-DNA alkyltransferase activity by O⁶-benzylguanine provides a means to evaluate the role of this protein in protection against carcinogenic and therapeutic alkylating agents. Proc. Natl. Acad. Sci. USA 87, 5368-5372.

[0138] Eshleman. J. R., Lang, E. Z., Bowerfind, G. K., Parsons, R., Vogelstein, B., Willson, J. K. V., Veigl, M. L., Sedwick;. W. D., and Markowitz, S. D. (1995). Increased mutation ate at the hprt locus accompanies microsatelite instability in colon cancer. Oncocene 10, 33-37.

[0139] Fang, W., and Modrich. P. (1993). Human strand-specific mismatch repair occurs by a bidirectional mechanism similar to that of the bacterial reaction. J. Biol. Chem. 268, 11838-11844.

[0140] Fishel, R., Ewel, A., and Lescoe, M. K. (1994). Purified human MSH2 protein binds to DNA containing mismatched nucleotides. Cancer Res. 54, 5539-5542.

[0141] Fishel, R., Ewel, A., Lee, S., Lescoe, M. K. and Griffith, J. (1994). Binding of mismatched microsatellite DNA sequences by the human MSH2 protein. Science 266, 1403-1405.

[0142] Fishel, R., Lescoe, M. K., Rao, M. R. S., Copeland. N. G., Jenkins, N. A., Garber, J., Kane, M., and Kolodner, R. (1993). The human mutator gene homolog MSH2 and its association with hereditary nonpolyposis colon cancer. Cell 75, 1027-1038.

[0143] Goldmacher, V. S., Cuzick , Jr., R. A., and Thilly, W. G. (1986). Isolation and partial characterization of human cell mutants differing in sensitivity to killing and mutation by methylnitrosourea and N-methyl-N′-nitro-N-nitrosoguanidine. J. Biol. Chem. 261, 12462-12471.

[0144] Griffin, S., and Karran, P. (1993). Incision at DNA G.T mispairs by extracts of mammalian cells occurs preferentially at cytosine methylation sites and is not targeted by a separate G.T binding reaction. Biochemistry 32, 13032-13039.

[0145] Griffin, S., Branch, P., Xu, Y. -Z., and Karran, P. (1994). DNA mismatch binding and incision at modified guanine bases by extracts of mammalian cells: Implications for tolerance to DNA methylation damage. Biochemistry 33, 4787-4793.

[0146] Han, H. -J., Yanagisawa, A., Kato, Y., Park, J. -G., Nakamura, Y. (1993). Genetic instability in pancreatic cancer and poorly differentiated type of gastric cancer. Cancer Research 53, 5087-5089.

[0147] Hartwell, L. (1992) Defects in a cell cycle checkpoint may be responsible for the genomic instability of cancer cells. Cell 71, 543-546.

[0148] Hemminki, A. Peltomäki, P, Mecklin, J. -P., Järvinen, H., Salovaara, R., Nyström-Lahti, M, de la Chapelle. A., and Aaltonen, L. A. (1994) Loss of the wild type MLH1 gene is a feature of hereditary nonpolyposis colorectal cancer. Nature Genetics 8, 405-410.

[0149] Holmes Jr., J., Clark. S., and Modrich, P. (1990) Strand-specific mismatch correction in nuclear extracts of human and Drosophila melanogaster cell lines. Proc. Natl. Acad. Sci. USA. 87, 5837-5841.

[0150] Hooper, M., Hardy, K., Handyside, A., Hunter, S., and Monk, M. (1987). HPRT-deficient (Lesh-Nyhan) mouse embryos derived from germline colonization by cultured cells. Nature, 326, 292-295.

[0151] Hughes, M. J., and Jiricny, J. (1992). The purification of a human mismatch-binding protein and association of its associated ATPase and helicase activities. J. Biol. Chem. 267, 23876-23882.

[0152] Jass, J. R., Stewart, S. M., Stewart, J., and Lane, M. R. (1994). Hereditary non-polyposis colorectal cancer: morphologies, genes and mutations. Mutat Res. 310. 125-133.

[0153] Jiricny, J. (1994). Colon cancer and DNA repair: have mismatches met their match? Trends in Genetics 10, 164-168.

[0154] Jiricny, J., Hughes, M., Corman, N., and Rudkin, B. B. (1988). A human 200-kDa protein binds selectively to DNA fragments containing G.T mismatches. Proc. Natl. Acad. Sci. USA 85, 8860-8864.

[0155] Karran, P., and Bignami, M. (1992). Self-destruction and tolerance in resistance of mammalian cells to alkylation damage. Nucl. Acids Res. 20,2933-2940.

[0156] Karran, P., and Bignami, M. (1994). DNA damage tolerance, mismatch repair and genome instability. BioEssays 16, 833-839.

[0157] Kat, A., Thilly W. G. Fang, W. -H., Longley, M. J., Li, G. -M., and Modrich, P. (1993). An alkylation-tolerant, mutator human cell line is deficient in strand-specific mismatch repair. Proc. Natl. Acad. Sci. USA 90, 6424-6428.

[0158] Koi, M., Umar, A., Chauhan, D. P., Cherian, S. P., Carethers, J. M. Kunkel, T. A., and Boland. C. R. (1994). Human chromosome 3 corrects mismatch repair defiency and microsatellite instability and reduces N-methyl-N′-nitro-N-nitrosoguanidine tolerance in colon tumor cells with homozygous hMLH1 mutation. Cancer Research 54, 4308-4314.

[0159] Kolodner, R, D., Hall. N. R., Lipford. J., Kane, M. F., Rao, M. R. S. Morrison, P., Wirth, L., Finan, P. J., Bum, J., Chapman. P., Earabino, C., Merchant, E., and Bishop, D. T. (1994). Structure of the human MSH2 locus and analysis of two Muir-Torre kindreds for msh2 mutations. Genomics 24, 516-526.

[0160] Kolodner, R, D., Hall, N. R., Lipford, J., Kane, M. F., Morrison, P., Wirth, L., Finan, P. J., Bum. J., Chapman, P., Earabino. C., Merchant, E., and Bishop, D. T. (1995). Structure of the human MLH1 locus and analysis of a large hereditary nonpolyposis colorectal carcinoma kindred for mlh1 mutations. Cancer Res.55, 242-248.

[0161] Kramer, B., Kramer, W., Williamson, M., and Fogel, S. (1989). Heteroduplex DNA correction in Saccharomyces cerevisiae is mismatch specific and requires functional PMS genes. Mol. Cell. Biol. 9, 4432-4440.

[0162] Kramer, W., Kramer, B., Williamson, M. S., and Fogel, S. (1989). Cloning and nucleotide sequence of DNA mismatch repair gene PMS1 from Saccharomyces cerevisiae: homology of PMS1 with procaryotic MutL and HexB. J. Bacteriology 171, 5339-5346.

[0163] Lazar, V., Grandjouan, S., Bognel, C., Couturier, D. Rougier, P., Bellet, D., and Bressac-de Paillerets, B. (1994). Accumulation of multiple mutations in tumour suppressor genes during colorectal tumorigenesis in HNPCC patients. Hum. Mol. Genet. 3, 2257-2260.

[0164] Leach, F. S., Nicolaides, N. C., Papadopoulos, N., Liu, B., Jen, J., Parsons, R. Peltomäki, P., Sistonen, P., Aaltonen, L. A., Nyström-Lahti. M., Guan, X. -Y. Zhang. L., Meltzer, P. S. Yu, J. -W., Kao, F. -T., Chen, D. I., Cerosaletti, K. M., Fournier, R. E. K., Todd, S., Lewis, T., Leach, R. J., Naylor, S. L., Weissenbach, I., Mecklin, J. -P., Järvinen, H., Petersen, G. M. Hamilton. S. R. Green, J., Jass, J., Watson, P., Lynch, H. T., Trent, J. M., de la Chapelle. A., Kinzler, K. W., and Vogelstein, B. (1993). Mutations of a mutS homolog in hereditary nonpolyposis colorectal cancer. Cell 75, 1215-1225.

[0165] Li, G. -M., and Modrich. P. (1995) Restoration of mismatch repair to nuclear extracts of H6 colorectal tumor cells by a heterodimer of human MutL homologs. Proc. Natl. Acad. Sci. USA 92, 1950-1954.

[0166] Liu, B., Nicolaides, N. C., Markowitz, S., Willson, J. K. V., Parsons, R. E. Jen, J., Papadopoulos, N., Peltomäki, P., de la Chapelle, A., Hamilton, S. R., Kinzler, K. W., and Vogelstein, B. (1995). Mismatch repair gene defects in sporadic colorectal cancers with microsatellite instablity. Nature Genetics 9, 48-55.

[0167] Liu, B., Parsons, R. E. Hamilton, S. R., Petersen, G. M., Lynch, H. T., Watson, P., Markowitz, S., Willson, J. K. V., Green, J., de la Chapelle, A., Kinzler, K. W., and Vogelstein, B. (1994). hMSH2 mutations in hereditary nonpolyposis colorectal cancer kindreds. Cancer Research 54, 4590-4594.

[0168] Loeb, L. A. (1994). Microsatellite instability: marker of a mutator phenotype in cancer. Cancer Research 54, 5059-5063.

[0169] Lynch, H. T. Smyrk, T. C., Watson, P., Lanspa, S. J., Lynch, J. F., Cavalieri, R. J., and Bolland, C. R. (1993). Genetics, natural history, tumor spectrum, and pathology of hereditary nonpolyposis colorectal cancer: an updated review. Gastroenterology 104, 1535-1549.

[0170] Michejda, C. J., Smith, Jr., R. H., and Kroeger Koepke, M. B. (1994). DNA alkylation by triazenes and related compounds. In DNA adducts: identification and biological significance, K. Hemminki, A. Dipple, D. E. G. Shuker, F. F. Kadlubar, D. Segerbäck, and H. Bartsch, eds. (IARC scientific publications and Oxford University press), pp. 323-337.

[0171] Miret, J. J., Milla, M. G., and Lahue, R. S. (1993). Characterization of a DNA mismatch-binding activity in yeast extracts. J. Biol. Chem. 268, 3507-3513.

[0172] Mironov, N. M., Aguelon, M. A. -M., Potapova, G. I., Omori, Y., Gorbunov, O. V., Klimenkov, A. A., and Yamasaki, H. (1994). Alterations of (CA)_(n) repeats and tumor suppressor genes in human gastric cancer. Cancer Res. 54, 41-44.

[0173] Modrich, P. (1991). Mechanisms and biological effects of mismatch repair. Annu. Rev. Genes. 25, 229-253.

[0174] Modrich, P. (1994). Mismatch repair, genetic stability, and cancer. Science 266, 1959-1960.

[0175] Mortensen, R. M., Conner, D. A. Chao, S., Geisterfer-Lowrance, A. A. T., and Seidman, J. G. (1992). Production of homozygous mutant ES cells with a single targeting construct. Mol. Cell. Biol. 12, 2391-2395.

[0176] Nassif, N., and Engels, W. (1993). DNA homology requirements for mitotic gap repair in Drosophila. Proc. Natl. Acad. Sci. USA 90, 1262-1266.

[0177] Nicolaides, N. C., Papadopoulos, N., Liu, B., Wei, Y. -F., Carter, K. -C., Ruben, S. M., Rosen, C. A., Haseltine, W. A., Fleischmann, R. D., Fraser, C. M., Adams M. D., Venter, J. C., Dunlop, M. G., Hamilton. S. R., Petersen, G. M., de la Chapelle. A., Vogelstein. B., and Kinzler, K. W. (1994). Mutations of two PMS homologues in hereditary nonpolyposis colon cancer. Nature 371, 75-80.

[0178] Nowak, R. (1994). Mining, treasures from ‘junk DNA’. Science 263, 608-610.

[0179] Orth, K., Hung, J., Gazdar, A., Bowcock, A., Mathis, J. M., and Sambrook, J. (1994). Genetic instability in human ovarian cancer cell lines. Proc. Natl. Acad. Sci. USA. 91, 9495-9499.

[0180] Palombo, F., Hughes, M., Jiricny, J., Truong, O., and Hsuan, J. (1994). Mismatch repair and cancer. Nature 367, 417.

[0181] Papadopoulos, N., Nickolaides, N. C., Wei, Y. -F., Ruben, S. M., Carter, K. C., Rosen, C. A., Haseltine W. A., Fleischmann, R. D., Fraser, C. M., Adams, M. D., Venter, J. C., Hamilton, S. R., Petersen, G. M., Watson, P., Lynch, H. T., Peltomäki, P., Mecklin, J. -P., de la Chapelle, A., Kinzler, K. W., and Vogelstein, B. (1994). Mutation of a mutL homolog in hereditary colon cancer. Science 263, 1625-1629.

[0182] Parsons, R., LI, G. -M, Longley, M. Modrich, P., Liu, B., Berk, T. Hamilton. S. R., Kinzler, K. W., and Vogelstein, B. (1995). Mismatch repair deficiency in phenotypically normal human cells. Science 268, 738-740.

[0183] Parsons, R., Li. G. -M., Longley, M. J., Fang, W., Papadopoulos, M., Jen, J., de la Chapelle. A. Kinzler. K. W., Vogelstein. B., and Modrich. P. (1993). Hypermutability and mismatch repair deficiency in RER⁺ tumor cells. Cell 75, 1227-1235.

[0184] Petit, M. A., Dimpfl, J., Radman, M. and Echols, H. (1991). Control of large chromosomal duplications in Escherichia coli by the mismatch repair system. Genetics 129, 327-332.

[0185] Prolla, T. A., Christie, D. -M., and Liskay, R. M. (1994). Dual requirement in yeast DNA mismatch repair for MLH1 and PMS1, two homologs of the bacterial mutL gene. Mol. Cell. Biol. 14, 407-415.

[0186] Prolla, T., Pang, Q., Alani, E., Kolodner, R. D., and Liskay, R. M. (1994). MLH1, PMS 1, and MSH2 interactions during the initiation of DNA mismatch repair in yeast. Science 265, 1091-1093.

[0187] Rayssiguier, C., Thaler, D. S. and Radman, M. (1989). The barrier to recombination between Escherichia coli and Salmonella typhimurium is disrupted in mismatch-repair mutants. Nature 342, 396-401.

[0188] Reenan, R. A. G., and Kolodner, R. (1992a). Isolation and characterization of two Saccharomyces cerevisiae genes encoding homologs of the bacterial HexA and MutS mismatch repair proteins. Genetics 132, 963-973.

[0189] Reenan, R. A. G., and Kolodner, R. (1992b). Characterization of insertion mutations in the Saccharomyces cerevisiae MSH1 and MSH2 genes: evidence for separate mitochondrial and nuclear functions. Genetics 132, 975-985.

[0190] Risinger, J. I., Berchuck, A., Kohler, M. F., Watson, P., Lynch, H. T., and Boyd, J. (1993). Genetic instability of microsatellites in endometrial carcinoma. Cancer Res. 53, 5100-5103.

[0191] Sambrook, J., Fritsch, E. F., and Maniatis, T. (1989) Molecular cloning, a laboratory manual. 2nd. edition. Cold Spring Harbor Laboratory Press.

[0192] Selva, E., New, L., Crouse, G., and Lahue, R. S. (1995). Mismatch correction acts as a barrier to homeologous recombination in Saccharomyces cerevisiae. Genetics 139, 1175-1188.

[0193] Shen, P., and Huang, H. V. (1986). Homologous recombination in Escherichia coli: Dependence on substrate length and homology. Genetics 112, 441-457.

[0194] Shibata, D., Peinado, M. A., Ionov, Y., Malkhosyan, S., and Perucho, M. (1994). Genomic instability in repeated sequences is an early somatic event in colorectal tumorigenesis that persists after transformation. Nature Genetics 6, 273-281.

[0195] Stephenson, C., and Karran, P. (1989). Selective binding to DNA base pair mismatches by proteins from human cells. J. Biol. Chem. 264, 21177-21182.

[0196] Strand, M., Prolla, T. A., Liskay, R. M., and Petes, T. D. (1993). Destabilization of tracts of simple repetitive DNA in yeast by mutations affecting DNA mismatch repair. Nature 365, 274-276.

[0197] Suzuki, H., Harpaz, N., Tarmin, L., Yin, J., Bell, J. D., Hontanosas, M., Groisman, G. M., Abraham, J. M., and Meltzer, S. J. (1994). Microsatellite instability in ulcerative colitis-associated colorectal dysplasias and cancers. Cancer Res. 54, 4841-4844.

[0198] Te Riele, H., Robanus Maandag, E., and Berns, A. (1992). Highly efficient gene targeting in embryonic stem cells through homologous recombination with isogenic DNA constructs. Proc. Natl. Acad. Sci. USA 89, 5128-5132.

[0199] Te Riele, H., Robanus Maandag, E., Clarke, A., Hooper, M., and Berns, A. (1990). Consecutive inactivation of both alleles of the pim-1 proto-oncogene by homologous recombination in embryonic stem cells. Nature 348, 649-651.

[0200] Thomas, D. C., Roberts, J. D., and Kunkel, T. A. (1991). Heteroduplex repair in extracts of human HeLa cells. J. Biol. Chem. 266, 3744-3751.

[0201] Umar, A., Boyer, J. C., and Kunkel, T. A. (1994a). DNA loop repair by human cell extracts. Science 266, 814-816.

[0202] Umar, A., Boyer, J. C., Thomas, D. C., Nguyen, D. C., Risinger, J. I., Boyd, J. Ionov, Y., Perucho, M. and Kunkel, T. A. (1994b). Defective mismatch repair in extracts of colorectal and endometrial cancer cell lines exhibiting microsatellite instability. J. Biol. Chem. 269, 14367-14370.

[0203] Varlet, I., Radman, M., and Brooks, P. (1990). DNA mismatch repair in Xenopus egg extracts: Repair efficiency and DNA repair synthesis for all single base-pair mismatches. Proc. Natl. Acad. Sci. USA 87, 7883-7887.

[0204] Varlet, I., Pallard, C., Radman, M., Moreau, J., and de Wind, N. (1994). Cloning and expression of the Xenopus and mouse Msh2 DNA mismatch repair genes. Nucl. Acids. Res. 22, 5723-5728.

[0205] Vogelstein, B., and Kinzler, K. (1993). The multistep nature of cancer. Trends in Bioch. Sci. 9, 138-141.

[0206] Wada, C., Shionoya, S., Fujino, Y., Tokuhiro, H., Akahoshi, T., Uchida, T., and Ohtani, H. (1994). Genomic instability of microsatellite repeats and its association with the evolution of chronic myelogenous leukemia. Blood 83, 3449-3456.

[0207] Waldman, A. S., and Liskay, R. M. (1988). Dependence of intrachromosomal recombination in mammalian cells on uninterrupted homology. Mol. Cell. Biol. 8, 5350-5357.

[0208] Wooster, R., Cleton-Jansen, A. -M., Collins, N., Mangion, J., Cornelis, R. S., Cooper, C. S., Gusterson, B. A., Ponder, B. A. J., von Deimling, A., Wiestler, O. D., Cornelisse, C. J., Devilee, P., and Stratton, M. R. (1994). Instability of short tandem repeats (microsatellites) in human cancers. Nature Genetics 6, 152-156.

[0209] Worth, Jr., L., Clark, S., Radman, M., and Modrich, P. (1994). Mismatch repair proteins MutS and MutL inhibit RecA-catalyzed strand transfer between diverged DNAs. Proc. Natl. Acad. Sci. USA 91, 3238-3241.

[0210] Young, J., Leggett, B., Gustafson, C., Ward, M., Searle, J., Thomas, L., Buttenshaw, R., and Chevenix-Trench, G., (1993). Genomic instability occurs in colorectal carcinomas but not in adenomas. Human Mutation 2, 351-353.

[0211] Legends

[0212]FIG. 1. Targeted disruption of the mouse Msh2 gene.

[0213] A: Top: schematic diagram of the mouse Msh2 protein. ATP binding and HTH: putative ATPase and helix turn helix domains, respectively (Varlet et al, 1994). The position at which the protein is disrupted by insertion of the Hyg marker is indicated. Bottom: the genomic Msh2 locus; indicated below is the targeting construct carrying the Hyg marker inserted into a unique SnaB I site within an exon sequence of Msh2. The positions of the external probes used for detection of targeting events are indicated.

[0214] B: The Msh2 locus in Msh2+/+ cell line wt-2, Msh2+/− cell line sMsh2-55 and Msh2−/− cell line dMsh2-9. Genomic DNA of these cell lines was digested with EcoRI and analyzed by Southern hybridization using probe 1 (FIG. 1A). Arrowheads indicate the positions of the wild type (wt, 20 kbp), and disrupted (ko, 7.5 kbp) alleles.

[0215]FIG. 2. Mismatch-binding activity in wild-type and Msh2-deficient cells.

[0216] Binding activity in extracts of Msh2+/+ cell line wt-2 and Msh2−/− cell line dmsh2-9 to double-stranded oligonucleotides containing a mismatch (G.T, G.A, G.G, or A.C), an extrahelical dinucleotide (Extrahelical TG), or an extrahelical 14-nucleotide loop (LOOP 14) and a Homoduplex was assessed using a gel retardation assay. Sequences of the oligonucleotides are given under Experimental Procedures. Arrowheads indicate the positions of mismatch-specific complexes. E2F: Binding to an oligonucleotide carrying an E2F site served as a positive control (Beijersbergen et al, 1995).

[0217]FIG. 3. Microsatellite stability in wild-type and Msh2-deficient cells.

[0218] Genomic DNA of 24 subclones derived from Msh2+/+ cell line wt-2 and Msh2−/− cell line dmsh2-9 was amplified by PCR using primer sets D7Mit17 and D14Mit15. Arrowheads indicate microsatellite alleles with a detectable length alteration.

[0219]FIG. 4. Tolerance of Msh2-deficient cells to the simple methylating agent MNNG.

[0220] ES cell lines wt-2 (sMsh2+/+), sMsh2-55 (Msh2+/−) and dMsh2-9 (Msh2−/−) were exposed to increasing amounts of MNNG for one hour in the presence of O⁶-benzylguanine. After four days of incubation, cells were trypsinized and counted. TABLE 1 Homologous recombination has lost dependence on sequence identity in Msh2 deficient ES cells. Homologous recombination vs total no. G418^(R) col. with Isogenic vs ES cells 129Rb-nec B/cRb-neo non-isogenic BALB/c (Msh2+/+)  1/68 (1.5%) 16/72 (22%)  15 x 129OLA (Msh2+/+) 33/94 (35%)  1/144 (0.7%)  50 x 129OLA (Msh2−/−) 42/185 (23%) 47/184 (26%) 0.9 x

[0221] TABLE 2 Phenotypes of Msh2 mutant mice. Tumor incidence Age Condition age Genotype (months) at birth type frequency** (months) Msh2+/− 8 healthy 0/110 Msh2+/+::Msh2−/−* 6 healthy Lymphoma 1/24 2   Msh2−/− 4-5 healthy Lymphoma 6/19 2-4 Histiocytic 1/19 3.5 sarcoma 

1. A method for modifying the genome of a eukaryotic cell by homologous recombination between a specific locus in the genome and a DNA sequence introduced into a cell in which the mismatch repair system has been genetically and/or transitorily inactivated and said sequence having up to 30% base sequence differences with respect to the target locus in the region where recombination can take place or said sequence differs from the target locus to such a level that homologous recombination is normally strongly impeded.
 2. A method according to claim 1 wherein the cell's mismatch repair system is inactivated by disruption of both copies of one of the genes essential for DNA mismatch repair.
 3. A method according to claim 1 wherein the cell's mismatch repair system is inactivated by introduction of antisense RNA constructs, driven by an appropriate promoter, thus functionally inactivating one of the genes essential for DNA mismatch repair.
 4. A method according to claim 1 wherein the cell's mismatch repair system is inactivated by introduction of modified antisense oligodeoxynucleotides, thus functionally inactivating one of the genes essential for DNA mismatch repair.
 5. A method according to claim 1 wherein the cell's mismatch repair system is inactivated by expression of a dominant negativity acting version of a gene involved in DNA mismatch repair.
 6. A method according to claim 1 wherein the cell's mismatch repair system is inactivated transitorily by saturation of the mismatch repair system by the introduction into the cell of DNA molecules carrying one or more mis- or unpaired bases.
 7. A method according to claims 1-6 wherein the target cell is an embryonic stem cell or a related cell type derived from any mammalian species, to be used to generate chimeric animals and chimeric animals that can transmit the mutated genome through the germ line.
 8. A method according to claims 1-6 wherein the target cell is a cell line derived from any, mammalian species and cultured in vitro.
 9. A method according to claims 1-6 wherein the target cell is derived from any organ of any mammalian species.
 10. A method according to claim 9 wherein the modified target cell can be reintroduced into the organism.
 11. A method according to claims 1-6 wherein the target cell is a plant cell.
 12. A method according to claims 1-6 wherein the target cell is derived from outbred organisms implicating that they contain nonidentical copies of homologous chromosomes.
 13. A method according to claim 1 wherein the targeting DNA at the region where homologous recombination can take place is derived from the same species as the target cell.
 14. A method according to claim 13 wherein the targeting DNA is modified in vitro at specific sites thus deliberately introducing sequence divergence.
 15. A method according to claim 1 wherein the targeting DNA at the region where homologous recombination can take place is derived from the same species as the target cell but is nonisogenic with the DNA of the target cell, the two sequences differing up to 5% at the nucleotide level.
 16. A method according to claim 1 wherein the targeting DNA at the region where homologous recombination can take place is derived from another species as the target cell, the targeting sequence and the target locus comprising up to 30% sequence divergence in the region where homologous recombination can take place.
 17. A method according to claim 16 wherein the targeting DNA is a chromosomal DNA fragment carried on a YAC or cosmid vector.
 18. A method according to claim 1 wherein the targeting DNA is an double- or single-stranded oligonucleotide consisting of 10-100 bases or basepairs of which one or several bases or basepairs differ from the target locus.
 19. A method according to claim 1 wherein the targeting DNA is constructed in such a way to allow the generation of deletions of chromosomal regions as follows: The targeting construct consists of any selectable marker gene flanked by two sequences which can recombine with chromosomal loci: one flanking sequence being identical or highly homologous (>95% sequence identity) to a sequence of the genome of the target cell, the other flanking sequence being a so called repetative sequence (e.g. Alu, LINE, SINE) of which numerous diverged copies are spread over the genome. The targeting construct consists of any selectable marker gene flanked by two sequences, one being a sequence that can act as a telomere, the other flanking sequence being a so called repetative sequence (e.g. Alu, LINE, SINE) of which numerous diverged copies are spread over the genome.
 20. A method to test inactivation of the cellular mismatch repair system related to claims 2, 3, 4, 5, 6 using a gene targeting assay involving comparison of the targeting efficiency of isogenic versus nonisogenic DNA targeting constructs.
 21. A method to test inactivation of the cellular mismatch repair system related to claims 2, 3, 4, 5, 6 using an assay involving intrachromosomal recombination between homologous but diverged DNA sequences.
 22. A method to test homologous recombination related to claims 13, 14, 15, 16, 17, 18, 19 using an Embryonic Stem cell line or any other cell line in which the mismatch repair system is inactived by disruption of both copies of the Msh2 gene or another mismatch repair gene.
 23. A method related to claims 8, 9, 10 in order to derive cells or cell lines from mismatch repair defecient mice carrying a disruption in both copies of the Msh2 gene or another mismatch repair gene. 